This invention relates generally to so-called marine "outboard motors" or marine outboard engine units each of which is a unit assembly of an internal-combustion engine, a power transmission mechanism, a screw or propeller, auxiliary devices, and a frame structure for supporting and holding together these parts. Such a unit assembly is hereinafter referred to as an "outboard engine". As is known, an outboard engine is dismountably mounted on the upper part of the transom of a boat hull or on some other stern part of the hull.
The above mentioned internal combustion engine in most outboard engines has been two-stroke-cycle gasoline engines, while others have been four-stroke-cycle gasoline engines, both types having one or more cylinders.
In an example of a known four-stroke-cycle engine (hereinafter referred to by the abbreviated term "four-cycle engine") used in an outboard engine, the crankshaft of the engine is in a forward position, that is, a position relatively near the stern of the boat hull when the outboard engine is in mounted state, while the camshaft is in an aft position, aft of the crankshaft. For this reason, when the boat hull is being driven, i.e., is under way, and assumes a bow-up attitude, the lubricating oil in the engine tends to move undesirably toward the cylinder head side of the engine, since the engine also tilts similarly as the hull. As a consequence, an undesirable "oil-rise" state arises.
Another typical feature of a known outboard engine is that the crankshaft of the engine is coupled directly to the extension drive shaft in the lower unit for driving the propeller via a bevel-gear speed-reduction mechanism functioning additionally to change the axis of rotation from vertical to horizontal. In general, the speed-reduction ratio of this mechanism is limited to a value of the order of 1/2 because of the necessity of restricting the bulk of the mechanism and its housing in order to reduce water resistance. Furthermore, the maximum rotational speed of the propeller is limited by the occurrence of cavitation.
As a consequence of the above described problems, the performance of a four-cycle engine in a conventional outboard engine has been unavoidably restricted by the necessity of keeping its rotational speed down, whereby it has not been possible to use a four-cycle engine of high-speed and high power output.